You probably have loose SIMMs or Bad RAM. Take the RAM out and replace it again. See if that solves the problem. Depending on your configuration you can remove part of the memory and just try some to determine the GOOD memor.

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You may want to check if your video card is seated the whole way in. While you have the case open to check that, take an pull your Ram chips out and take a couple and place them into different sockets and try again to restart. If you have an old video card, try replacing your existing one with the old one to test to see if it's the video card.

How to fix display issue, screen flickering issue when I plug in
power cord to the machine.
Before I start explaining the solution lets check out once the issue how it looks like
after I connect the power cord. most of you also have faced this…